1. Field
The present disclosure relates generally to a transmitter used for transmitting orthogonal frequency-division multiplexing (OFDM) modulated signals. More specifically, the present disclosure relates to a transmitter equipped with an adaptive power amplifier driver that is capable of achieving high efficiency when pre-amplifying OFDM-modulated signals.
2. Related Art
Orthogonal frequency-division multiplexing (OFDM) technology has become more and more popular in recent years because of its many advantages, including frequency efficiency and robustness against frequency-selective channel fading in the tough wireless environment. During the past decade, OFDM has become the basis of many standards, such as Wi-Fi, Worldwide Interoperability for Microwave Access (WiMAX), Digital Video Broadcasting (DVB), Long Term Evolution (LTE), TV White Space (TVWS), etc.
However, OFDM also suffers from some drawbacks. One important problem is the high peak-to-average power ratio (PAPR) of the transmitted signal. The high peak can result in saturation of the power amplifiers (PAs), leading to non-linear signal distortion. To prevent the non-linear distortion, conventional approaches rely on keeping the power amplifier working in the linear range by backing-off the output power of the amplifier entirely to accommodate the high peaks. Such approaches can result in either a low signal-to-noise ratio (SNR) or an oversized and inefficient power amplifier (PA). Moreover, to ensure the linearity of the transmitted signals, other transmitter components, such as low-pass filters (LPFs), variable gain amplifiers (VGAs), mixers, and PA drivers, have to work in the high-linearity, low-power-efficiency mode.